This invention relates generally to production of modified polypropylenes and more particularly to a process for producing modified polypropylenes or heteroblock copolymers of propylene having desirable characteristics such as excellent surface lustre and high bending rigidity, which are inherent characteristics of isotactic polypropylenes and having, moreover, excellent resistance to impact.
Still more specifically, invention inventio relates to a novel process for producing heteroblock copolymers of propylene which comprises a stage wherein, through the use of a stereospecific catalyst for olefin polymerization, preferably a catalyst comprising (1) a titanium halogen compound and (2) an organoaluminum compound, one cycle or repeated cycles of a combination of polymerization of propylene by itself and copolymerization of a minute quantity of ethylene and propylene are carried out, and a polymer having an ethylene/propylene random copolymer segment containing a minute quantity of ethylene is formed in the polypropylene and a stage wherein an ethylene/propylene mixture of an ethylene concentration above 50 percent by weight is polymerized, and an ethylene/propylene copolymer segment having a high ethylene content is caused to undergo block copolymerization.
While isotactic polypropylenes, in general, are excellent resins having properties such as high rigidity, high resistance to heat, and excellent surface lustre, they have low resistance to impact and are difficult to use in applications where they are subject to mechanical impact or shock (Reference is made to Comparison Example 11).
For this reason, there have heretofore been a number of proposals relating to the improvement of the impact resistance of these polymers. For example, processes such as (1) a process wherein a polyethylene, an ethylene/propylene rubber, or both are mechanically blended and (2) a process wherein propylene and ethylene or another olefin are stepwisely polymerized thereby to form a block copolymer are known. Particularly in relation to the process (2), there is a known process in which block copolymers wherein ethylene/propylene copolymer segments containing ethylene in various concentrations are connected in various proportions are produced by two-stage polymerization or three-stage polymerization.
While the impact resistance of the product is undoubtedly improved by these procedures, surface lustre, which is one of the characteristics of isotactic polypropylene, is lost as a result. Thus, these processes have not been completely satisfactory in all cases for producing modified polypropylene for formed articles of practically balanced quality retaining simultaneously both excellent lustre and high rigidity and, moreover, having excellent impact resistance.
For example, processes for producing modified polypropylene by two-stage polymerizations are disclosed in Japanese Patent Publication Nos. 15535/1964 (U.S. Pat. No. 3,629,368) and 8207/1972 and Patent Laid Open No. 25291/1972. We have tried out these processes, and, as a result, obtained ethylene/propylene copolymers (e.g., Comparison Example - 7 set forth hereinafter) which were not very satisfactory in surface lustre in comparison with polypropylene.
Furthermore, processes comprising three-stage polymerization are disclosed in Japanese Patent Publication Nos. 20621/1969, U.S. Pat. No. 3,670,053 specification, and Japanese Patent Laid Open No. 2578/1973. We have tried out these processes also, and, as a result, obtained ethylene/propylene copolymers (e.g., Comparison Examples 8 and 9 set forth hereinafter) which also were not very satisfactory on the point of surface lustre and impact strength.
Accordingly, since impact-resistant polypropylenes produced by these known processes ordinarily have surface lustres of from 30 to 45, which are far inferior to those of homopolypropylenes, their uses are limited to partial industrial fields such as containers and internal materials for electronic components. Therefore, there has been a need for the development of polypropylenes having excellent impact resistance and surface lustre which can be used as external materials for household goods, parts of electric appliances, and the like particularly requiring good external appearance with surface lustres higher than 55.